1. Field of the Invention
The present invention relates generally to an illumination system of the type used in lithographic apparatus for semiconductor wafer manufacture.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging the pattern using a UV radiation beam onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of substrate through the use of a reflection system. The interference causes lines to be formed on at the target portion of the substrate.
Lithography apparatus typically includes an alignment system that detects positions of alignment marks on a wafer and for aligning the wafer to ensure accurate exposure from a mask. In a double patterning process, a first resist layer having an alignment mark is coated with a second resist layer prior to etching the first and second resist layers. In double patterning systems, the alignment marks in the first resist layer are used to align a mask and expose the second resist layer. However, the alignment mark in the first resist layer may be obliterated or may provide poor contrast and weak diffraction orders when illuminated at an alignment wavelength since it is coated over with the second resist layer. Methods and systems are needed to overcome the above alignment deficiencies in double patterning systems.